400 sq feet is just about the right size for a single person. It fit my own work prototypes that I refuse to part with, my vintage finds, and the best of IKEA basics. While it's a fifth floor walk up, with courtyard facing light, the rent is well within my range for a great and centrally located neighborhood. The courtyard view makes this feel like a quiet oasis in the middle of midtown Manhattan, and having a kitchen that isn't visible from your bed is hard to find in Manhattan studios.

Three of My Best Tips and Tricks for Creating a Successful Small Space:

1) Layout in zones. I broke it up into a dressing/landing strip, work/eat, lounge, and sleep areas. In an open studio space, you can define zones through paint. It helps minimize clutter, by keeping everything in its place.
2) Maximize each piece. Let your furniture do double duty. Essential for tiny apartments. The bar is both a workstation and dining area, and the console is a bar with an ottoman nested under it for extra company. The nightstand is also an end table with a trash bin nesting under it. The swing lamp on the wall lights the living area and gives the plants much needed artificial light supplements.
3) Splurge on storage. If you can have one expensive thing, let it be a good dresser or closet system. It will keep you organized and pile free.